Wireless frequency spectrum is controlled by government bodies. These government bodies allocate the frequency spectrum to particular wireless operators, and place conditions on how the frequency spectrum is used. For example, allocated frequency spectrum is typically required to operate either in a time division duplex (TDD) or frequency division duplex (FDD) mode. In a system that operates in a TDD mode, the uplink and downlink channels share the same frequency band, but are transmitted and received by the base station during mutually exclusive periods of time. In a system that operates in an FDD mode, the uplink and downlink channels are transmitted simultaneously on different frequency bands.
The 2.5 GHz frequency spectrum currently consists of TDD spectrum, and is being partially re-banded to support FDD. Currently, Code Division Multiple Access (CDMA) and the IEEE 802.16 standard, which uses Orthogonal Frequency Division Multiple Access (OFDMA), both support TDD and FDD modes. Typically, systems which use CDMA or the IEEE 802.16 standard operate in either a TDD or FDD mode.
TDD technologies are currently capable of flexible channel bandwidth support, such as Time Division-Code Division Multiple Access (TD-CDMA) supporting 5 MHz or 10 MHz with a chip rate change, or Worldwide Interoperability for Microwave Access (WiMAX) wireless technology supporting from 1.25 MHz to 20 MHz through either increasing tone spacing or Fast Fourier Transform (FFT) size in a scalable Orthogonal Frequency Division Multiple Access (OFDMA) system. In other words, these technologies are capable of supporting multiple bandwidths. Furthermore, the TDD nature of the technology allows a flexible allocation of radio resources between uplink and downlink through varying the time dedicated to each link.
The flexible allocation of radio resources between uplink and downlink is useful for asymmetric communications, such as data communications. In data systems, downlink traffic usually requires more control channel overhead, larger sized data traffic requests in the form of file downloads etc., broadcast/multicast services from base station to several mobile stations, or other requests that require higher resources (time or frequency) for the downlink, as compared to the uplink. TDD systems accommodate this situation by allowing a flexible downlink to uplink ratio (e.g. 2:1).
Compared to TDD systems, FDD systems are more effective in utilizing the spectrum, and avoid the need for stringent synchronization, as required between the downlink and uplink portion of a TDD frame in neighboring cell sites. This FDD characteristic greatly helps to avoid the interference caused by high power downlink transmission on neighboring low power uplink transmission. However, compared to TDD systems, conventional FDD systems cannot accommodate the asymmetrical nature of downlink and uplink traffic.